Alice Ohlson Yale University

Jets at RHIC 15 July 2013Jets at RHIC -- Alice Ohlson2 Hard-scattered partons fragment into collimated “jets” of hadrons By studying jets we can investigate interactions of the high-p T partons with the Quark Gluon Plasma We look for… Suppression of the number of jets (compared to p+p or p+A) Modification of the p T or angular distributions of jet fragments Au+Au collisions at √s NN = 200 GeV

High-p T suppression 3 Single-particle inclusive measurements → suppression of high-p T hadrons in central Au+Au events Correlations w.r.t. high-p T hadrons → suppression of particles associated with jet peak Jets at RHIC -- Alice Ohlson15 July 2013 PRL 101 (2008) PRL 91 (2003)

Jet Reconstruction Algorithms The goal: Find clusters of particles, determine the direction and energy of the original parton Several options: k T, anti-k T, Gaussian filter, etc. Main challenge: subtracting the combinatoric heavy ion background, accounting for fluctuations 15 July 2013Jets at RHIC -- Alice Ohlson4 Gaussian filter arXiv: [nucl-ex] k T & anti-k T JHEP04 (2008) 063

Jet Spectra at RHIC For the first time → Full jet reconstruction in a heavy ion environment Different methods of jet reconstruction, background subtraction, fake- jet rejection 15 July 2013Jets at RHIC -- Alice Ohlson5 QM 2009

Jet R AA at RHIC Reconstructed jets demonstrate suppression Need to use similar techniques for direct comparison, further work is necessary to understand background fluctuations 15 July 2013Jets at RHIC -- Alice Ohlson6 QM 2009 jets

Jet Spectra at the LHC ALICE, CMS, and ATLAS have produced jet spectra for 30 < p T jet < 300 GeV/c Jet R AA shows suppression for wide range in jet p T 15 July 2013Jets at RHIC -- Alice Ohlson7 ALICE, HQ 2012

Dihadron Correlations 15 July 2013Jets at RHIC -- Alice Ohlson8 associates trigger hadron PRL 91 (2003) Investigate correlations of all “associated” particles with a “trigger”

γ- hadron Correlations 15 July 2013Jets at RHIC -- Alice Ohlson9 associates trigger photon γ-jet pairs produced in q+g → q+γ Photons do not lose energy in the medium, therefore p T photon ≈ p T parton

Awayside hadron yields Modified fragmentation function in Au+Au compared to p+p In narrow cone (|Δφ-π| < π/6): suppression (I AA < 1) of high-z T hadrons, no corresponding enhancement at low z T When integration region is expanded (|Δφ-π| < π/2), low-z T enhancement is observed 15 July 2013Jets at RHIC -- Alice Ohlson10 PHENIX, arXiv: [nucl-ex] Accepted to PRL |Δφ-π| < π/6 |Δφ-π| < π/3 |Δφ-π| < π/2 yield in Au+Au yield in p+p I AA = ξ = ln(1/z T ) z T = p T hadron /p T photon high z T low z T

Jets at the LHC CMS result  Energy is distributed to very wide angles (ΔR > 0.8 ~ π/4) Similar conclusions for CMS A J and PHENIX γ-jet measurements Where does the “missing” energy go? 15 July 2013Jets at RHIC -- Alice Ohlson11 ΔR>0.8

Jet-hadron Correlations Jet-hadron correlations have increased kinematic reach compared to dihadron correlations, allow for more precise determination of parent parton energy 15 July 2013Jets at RHIC -- Alice Ohlson12 associates reconstructed trigger jet

Jet-hadron Correlations Intentionally impose a bias towards unmodified trigger jets! (surface bias?) E T > 6 GeV in a single BEMC tower (Δφ x Δη = 0.05 x 0.05) Anti-k T (R = 0.4) using tracks/towers with p T > 2 GeV/c HT trigger requirement and constituent p T cut Reduce effects of background fluctuations Comparison to p+p is more straightforward Trigger (nearside) jet population is highly- biased Used to assign uncertainties to shape of background (v 2 and v 3 ) and trigger jet energy scale 15 July 2013Jets at RHIC -- Alice Ohlson13 Recoil (awayside) jet fragmentation is unbiased

Awayside Gaussian Widths Awayside widths suggest jet broadening to large angles at low-p T (but highly-dependent on assumed v 3 modulation) Further information is needed about v 2 jet, v 3 jet (possible correlation of jets with reaction plane / participant planes)… 15 July 2013Jets at RHIC -- Alice Ohlson14 STAR, arXiv: [nucl-ex] Submitted to PRL

Awayside Energy Balance Suppression of high-p T associated hadron yield is in large part balanced by low-p T enhancement. 15 July 2013Jets at RHIC -- Alice Ohlson15 p T jet,rec (GeV/c) ΔB (GeV/c) Uncertainties due to: detector effects v 2 and v 3 jet energy scale STAR, arXiv: [nucl-ex] Submitted to PRL

From Qualitative Consistency to Quantitative Conclusions Need to assess biases in measurements at RHIC and the LHC… Dihadron correlations: high-p T hadron → hard fragmentation γ-hadron correlations: no geometrical bias? Jet-hadron correlations: HT trigger and high-p T constituent cut → hard fragmentation CMS A J : Two reconstructed jets with high energy Need to do the same measurements at each experiment A J at RHIC Jet-hadron correlations at LHC 15 July 2013Jets at RHIC -- Alice Ohlson16

Select events containing pairs of back-to-back high-p T hadrons (likely due to dijet production) Investigate distributions of associated hadrons with respect to both trigger hadrons 2+1 Correlations 15 July 2013Jets at RHIC -- Alice Ohlson17 associates primary trigger (trig1) dijet trigger (trig2)

Symmetric Triggers No significant difference between Au+Au and d+Au No significant difference between near-side and away-side. Are we sampling surface-biased/unmodified dijets? Or dijets in which both jets lose similar amounts of energy? 15 July 2013Jets at RHIC -- Alice Ohlson18 5 < p T trig1 < 10 GeV/c 4 < p T trig2 < p T trig1 1.5 GeV/c < p T assoc < p T trig1 PRC 83 (2011) Near-sideAway-side ■ Au+Au ● d+Au

Asymmetric Triggers Still no large shape difference between near- and away-sides, or between Au+Au and d+Au. Energy imbalance indicates slight softening of awayside peak Compare/contrast to dihadron and jet-hadron results. 15 July 2013Jets at RHIC -- Alice Ohlson19 10 < E T trig1 < 15 GeV (in BEMC) 4 < p T trig2 < 10 GeV/c (in TPC) 1 < p T assoc < 10 GeV/c PRC 87 (2013) Near-sideAway-side ■ Au+Au ● d+Au

Jet-Hadron and 2+1 Correlations Require a high-p T hadron ~180 º away from reconstructed trigger jet 15 July 2013Jets at RHIC -- Alice Ohlson20 associates reconstructed trigger jet dijet trigger (trig2)

Jet-Hadron and 2+1 Correlations Select unmodified jets with p T hadron > 4 GeV/c requirement. 15 July 2013Jets at RHIC -- Alice Ohlson21 no trig2 requirement 10 < p T jet < 20 GeV/c | φ jet – φ trig2 | > π – 0.2 Low p T assoc 1.5 < p T assoc < 2 GeV/c High p T assoc 6 < p T assoc < 8 GeV/c STAR, WWND 2011 p T trig2 > 2 GeV/cp T trig2 > 4 GeV/c

Jet v 2 at STAR Correlation between jet axis and event plane Jet v 2 {FTPC EP} is non-zero → more jets reconstructed in-plane than out-of-plane → evidence of pathlength-dependence of parton energy loss Jet v 2 ≈ HT v 2 → bias towards unmodified jets largely driven by HT requirement 15 July 2013Jets at RHIC -- Alice Ohlson22 Jet Definition: HT trigger E T > 5.5 GeV constituent p T cut = 2 GeV/c |η jet | < 0.6 ● v 2 {TPC EP} (|η| < 1) ● v 2 {TPC EP} (2.8 < |η| < 3.7) STAR Preliminary STAR, QM 2012

Jet v 2 at ATLAS Jet v 2 measured for 45 < p T jet < 210 GeV/c, R = 0.2 Also observed v 2 jet > 0 Different kinematic range and biases than STAR measurement → different trend with p T jet 15 July 2013Jets at RHIC -- Alice Ohlson23 ATLAS, arXiv: [hep-ex] Submitted to PRL

RHIC/LHC Complementarity RHIC + LHC can cover a huge kinematic range of jets… ~5-10 GeV γ-jets in PHENIX ~10-40 GeV reconstructed jets in STAR ~ GeV jets in ALICE ~ GeV jets in CMS/ATLAS 15 July 2013Jets at RHIC -- Alice Ohlson24 STAR is working to improve their jet p T spectrum measurement Recent advancements in understanding the details of jet reconstruction Similar methods as ALICE for direct comparison A new high-statistics dataset → jets beyond 50 GeV sPHENIX upgrade will allow additional reach

Conclusions & Final Thoughts Complementary analyses demonstrate jet quenching at RHIC & LHC Softening and broadening of jets which traverse the medium in heavy ion collisions compared to p+p collisions It is necessary to understand biases present in analyses At RHIC energies, a p T > 4 GeV/c hadron requirement selects mostly unmodified jets What about at LHC energies? Necessary to do similar measurements with similar techniques among RHIC experiments and at the LHC Complementarity between RHIC and LHC measurements provide information about parton energy loss over a wide kinematic range 15 July 2013Jets at RHIC -- Alice Ohlson25 φ η p T per unit grid cell (GeV/c) STAR Preliminary

15 July 2013Jets at RHIC -- Alice Ohlson26

Hard-scatterings occur in the early stages of the collision Recoiling partons fragment into clusters of hadrons, known as “jets.” Use jets as probes of the medium. Jets PRL 97 (2006) Jets in p+p are well-described by pQCD 15 July Jets at RHIC -- Alice Ohlson

Jet Reconstruction Anti-k T algorithm – sequential recombination [PLB 641 (2006) 57] Start with high-p T particles For each pair of particles (i,j), calculate d ij = min(1/p Ti 2, 1/p Tj 2 )((Δy) ij 2 +(Δφ) ij 2 )/R 2 If d ij < 1/p Ti 2, merge the particles, else call particle i a jet Repeat until all particles are clustered R is resolution parameter → characteristic jet radius M. Cacciari, G. P. Salam, G.Soyez, JHEP (2008) 15 July Jets at RHIC -- Alice Ohlson

STAR (Solenoidal Tracker at RHIC) 15 July 2013Jets at RHIC -- Alice Ohlson29 charged particle tracking & PID neutral energy detection Zero Degree Calorimeter 0 < φ < 2π |η| < 1 Barrel

Jet-Hadron correlations 15 July 2013Jets at RHIC -- Alice Ohlson30

Similar trigger jet population in Au+Au and p+p due to p T cut and HT trigger requirements Compare recoil jet spectrum in Au+Au and p+p Suppression of recoil jet in Au+Au Due to softening and/or broadening outside of jet cone Consistency between correlations and inclusive measurements. Dijet Coincidence Rate 15 July 2013Jets at RHIC -- Alice Ohlson31 Trigger Jet: R = 0.4 p T,cut = 2 GeV/c p T jet > 20 GeV/c Recoil Jet: R = 0.4 p T,cut = 2 GeV/c STAR, QM 2009 p+p Au+Au (0-20%) Au+Au (0-20%) unfolded

Asymmetric Triggers Dijet imbalance indicates slight softening of awayside peak Still no significant shape difference between near- and away-sides, or between Au+Au and d+Au. Compare/contrast to dihadron and jet-hadron results. 15 July 2013Jets at RHIC -- Alice Ohlson32 10 < E T trig1 < 15 GeV (in BEMC) 4 < p T trig2 < 10 GeV/c (in TPC) 1 < p T assoc < 10 GeV/c ○ 8 < E T trig1 < 10 GeV □ 10 < E T trig1 < 15 GeV Relative dijet imbalance Δ(ΣE T ) Au+Au – Δ(ΣE T ) d+Au PRC 87 (2013) 44903

What is jet v 2 ? 15 July 2013Jets at RHIC -- Alice Ohlson33

Artificial Sources of Anisotropy 15 July 2013Jets at RHIC -- Alice Ohlson34

Background Fluctuations 15 July 2013Jets at RHIC -- Alice Ohlson35

Jet - Event Plane bias 15 July 2013Jets at RHIC -- Alice Ohlson36

Jet v 2 vs. Reconstructed Jet p T 15 July 2013Jets at RHIC -- Alice Ohlson37

Does the recoil jet hit the FTPC? 15 July 2013Jets at RHIC -- Alice Ohlson38